plugins/driver: update driver interface to support streaming stats
client/tr: use streaming stats api
TODO:
* how to handle errors and closed channel during stats streaming
* prevent tight loop if Stats(ctx) returns an error
drivers: update drivers TaskStats RPC to handle streaming results
executor: better error handling in stats rpc
docker: better control and error handling of stats rpc
driver: allow stats to return a recoverable error
Re-export the ResourceUsage structs in drivers package to avoid drivers
directly depending on the internal client/structs package directly.
I attempted moving the structs to drivers, but that caused some import
cycles that was a bit hard to disentagle. Alternatively, I added an
alias here that's sufficient for our purposes of avoiding external
drivers depend on internal packages, while allowing us to restructure
packages in future without breaking source compatibility.
